1. Field of the Invention
The present invention relates to a novel highly sensitive photosensitive lithographic printing plate and, more specifically, it relates to a highly sensitive photosensitive lithographic printing plate particularly suitable to projection exposure.
2. Discussion of the Background
Heretofore, exposure of a photosensitive lithographic printing plate has been conducted by bringing a photosensitive printing plate into close contact with an equal size silver halide film and, subsequently, irradiating with intense UV-radiation. In recent years, a projection exposure device for enlarging an image on original film has been used, by utilizing an optical system and printing on a photosensitive lithographic printing plate.
The magnifying ratio in the projection exposure method is about 3 to 10 times, by which the area of the silver halide film used can be reduced to 1/9-1/100 as compared with the conventional contact method. Further, since there is no requirement for close contact of a printing frame, the time required for vacuum contact (usually about 30 sec to 2 min) can be eliminated in the projection exposure method.
Furthermore, since original film (usually microfilm) is small in size and lightweight, films can be selected and aligned automatically, thereby eliminating laborious selection and alignment operations.
A photosensitive layer having a double layer structure is disclosed in Japanese Patent Laid-Open Sho 53-145706, 57-38481 and 58-2847, but the structure disclosed in each of these references is intended for development with tap water and none mentions increasing sensitivity by 10-50 times which is required for projection exposure.
For instance, Japanese Patent Laid-Open Sho 53-145706 and 57-38431 each relate to a lithographic plate having a double layer structure containing a diazo photosensitive material. The thickness of the lower layer is disclosed as being from 0.5 to 21.5 mg/dm.sup.2, while the thickness of the upper layer is from 0.5 to 32.2 mg/dm.sup.2. Further, the diazo concentration in the lower layer is from 1 to 99%, while the diazo concentration in the upper layer is from 2 to 90%, and the ratio of the thicknesses and the ratio of the diazo concentrations between the upper layer and the lower layer are not mentioned. In Japanese Patent Laid-Open Sho 58-2847, it is stated that the thickness of the lower layer is from 1 to 30 mg/dm.sup.2, while the thickness of the upper layer is from 0.5 to 40 mg/dm.sup.2, and that the diazo concentration in the lower layer is from 1 to 50%, while the diazo concentration in the upper layer is from 1 to 50%.
Use of projection exposure systems has not yet been popularized, in spite of its various merits, partly because the photosensitive printing plate lacks sensitivity.
A lithographic printing plate for contact exposure requires about 30 sec to 2 min for vacuum contact before the light exposure, as well as about the same amount of time for exposure itself. However, in order to effectively utilize a projection exposure device, it is only necessary that the exposure time be about 2 to 5 sec. That is, there has been a demand for the development of a lithographic printing plate of higher sensitivity as compared with the conventional contact exposure printing plate, more specifically, a lithographic printing plate with UV irradiation energy requirements of 5 to 25 mj/cm.sup.2 and having a sensitivity as high as 10 to 50 times compared with the irradiation energy of existing photosensitive lithographic printing plates for contact exposure (200 to 500 mj/cm.sup.2). Further, micro-projection exposure devices have been introduced mainly into the field of monochromatic printing, in which an additive-type plate is used in addition to a subtractive type plate. It is desirable to use developers and automatic developing machines, which are now used for contact exposure, for projection exposure also.